/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution
{
public:
    bool hasCycle(ListNode *head)
    {
        unordered_set<ListNode *> sum;
        while (head != nullptr)
        {
            if (sum.count(head))
                return true;
            sum.insert(head);
            head = head->next;
        }
        return false;
    }
};
// 力扣官方的快慢指针.
class Solution
{
public:
    bool hasCycle(ListNode *head)
    {
        if (head == nullptr || head->next == nullptr)
            return false;
        ListNode *slow = head;
        ListNode *fast = head->next;
        while (slow != fast)
        {
            if (fast == nullptr || fast->next == nullptr)
                return false;
            slow = slow->next;
            fast = fast->next->next;
        }
        return true;
    }
};
